1. Field of the Invention
An aspect of the present invention relates to an optical pickup actuator, and, more particularly, to a quadaxial driving optical pickup actuator, which is capable of tilt driving in a tangential direction.
2. Description of the Related Art
An optical pickup is a core element of optical memory systems such as optical recording and/or reproducing apparatuses, and requires an actuator which drives an objective lens. The actuator generally has a moving coil that intersects a magnetic field of a permanent magnet and performs motion with one-degree-of-freedom (a translational motion) to accurately move an objective lens to a desired position.
However, while an optical recording and/or reproducing apparatus records and/or reads information on and/or from a disc, if the disc is bent or flexed, an optical signal generated from the disc is degraded. In general, the degradation of an optical signal caused by a tilt of a disc is compensated by measuring the tilt of the disc by a tilt sensor with respect to an optical pickup and tilting an objective lens by an actuator that is capable of tilt driving to correct the tilt of the objective lens relative to the disc.
Japanese Published Patent Application No. 2000-195078 disclosed an actuator capable of tilt driving in three axial directions in order to compensate for the degradation of an optical signal by the tilt of a disc. FIG. 1 is a schematic view of an optical pickup actuator disclosed in Japanese Published Patent Application No. 2000-195078 and a partial perspective view of a moving unit 8, a tilt spring 9 and a suspension wire 4 thereof.
The moving unit 8 includes a lens holder 2 supporting an objective lens 1, and permanent magnets 3a, 3b, 3c and 3d. The objective lens 1 and the permanent magnets 3a, 3b, 3c and 3d are fixed to the lens holder 2.
One end of the suspension wire 4 is fixed to the moving unit 8, and the other end thereof is fixed to the tilt spring 9. The tilt spring 9 is combined with a suspension holder (not shown). According to FIG. 1, reference numeral 9a denotes a central transformation axis which the tilt spring 9 is deformed around, −Foa, −Foc, +Fob and +Fod are the driving forces obtained when an electrical current is applied to a focusing coil (not shown).
FIGS. 2A through 2C are front views of the moving unit 8 from the direction of arrow Vt of FIG. 1. FIG. 2A illustrates a state (initial state) when the driving forces do not act on the moving unit, and FIGS. 2B and 2C illustrate respective tangential tilt states of the moving unit 8. In FIGS. 2B and 2C, ±Mt is an axially rotational moment in the tangential tilt direction acting on the moving unit 8. The resultant force of the driving forces −Foa, −Foc, +Fob and +Fod acts on the moving unit 8 as the moment +Mt or −Mt of the x-axial rotational force.
As shown in FIGS. 2B and 2C, the suspension wire 4 may be in a tensile or compressive state according to the moment of a force, and the tilt spring 9 is deformed around a central transformation axis 9a thereof, such that a fixed supporter 4a of the suspension wire 4 is displaced in a tensile or compressive direction. As a result, the moving unit 8 is movably supported in the tangential tilt direction, in proportion to an amount of a displacement of the fixed supporter 4a.
In the conventional actuator described above, the tilt spring 9 is disposed to support the fixed supporter 4a of the suspension wire 4 so as to be displaced in the tensile or compressive direction, thereby driving the moving unit 8 in quadaxial directions (i.e., not only a in a focusing direction, a tracking direction, and a radial direction but also in a tangential tilt direction relative to the base of the actuator and to an information storage medium being acted upon).
Since the conventional actuator includes the tilt spring 9 being connected to the suspension holder and flexibly deforming a plate to which the suspension wire is fixed, the moving unit including the objective lens is movable in the tangential tilt direction in addition to the radial tilt direction. That is, in the conventional actuator, the quadaxial driving in the radial tilt direction and the tangential tilt direction (in addition to the focusing direction and the tracking direction) is possible by relatively greatly reducing the torsion coefficient in the radial direction and the torsion coefficient in the tangential direction, based on a method of deforming the plate as a fixed requirement. Also, an electromagnetic circuit capable of the quadaxial driving is constituted in a moving magnet structure as shown in FIG. 1. Thus, a wiring configuration may be relatively easily realized.
However, since the conventional tilt actuator causes Y-translation along a Y-axis, or yawing-related sub-resonance on the focusing or tracking characteristics, actuator performance may deteriorate relatively seriously. Here, the Y-axis is defined as a direction that is perpendicular to the X-axis shown in FIG. 1. The Y-translation is a translational mode in an unnecessary direction, i.e., in the direction perpendicular to the focusing driving direction and the tracking driving direction, in addition to the required translational motion in the focusing direction and the tracking direction.
Consequently, the conventional tilt actuator does not satisfy the condition that any unnecessary motions must not occur, except for the four motions of focusing, tracking, radial tilt and tangent tilt, by maximally decreasing the natural frequency related to a pitching mode which is a relevant vibration mode enabling the tangential tilt motion, and maximally increasing the natural frequency in the Y-translation or yawing mode.